The present invention relates to an image forming apparatus for forming an image received from an original document or an image signal on a sheet of paper, and more particularly, to an image forming apparatus for forming a multicolored, unicolored or monochromic image by using different-colored developers.
In order to print an intended image with the image forming apparatus such as a copy machine, printer or facsimile, first, a photosensitive material is exposed according to image information obtained from an original document or a predetermined image source, thereby forming a latent image. Then, a developer (e.g., toner) sticks to the portion where the latent image is formed, in order to develop the image. The developed image is transferred and fixed on a sheet of paper.
To form a color image, usually three (or four) developers for yellow, magenta and cyan (sometimes including black) are used. Being contained in respective developing devices, different-colored developers are sequentially selected, adhered to the portion of latent image of the photosensitive material, and transferred onto a sheet of paper. A color image forming apparatus with a plurality of developers respectively containing the different-colored developers is divided into a fixed type and a rotary type according to the arrangement of the developing devices.
In the fixed type, a plurality of developing devices are fixedly disposed near a photoconductive drum so that a selected developing device operates to apply its developer to the photoconductive drum at its own position (see U.S. Pat. No. 4,063,724).
In the rotary type, the developing devices are radially incorporated in a rotator rotatably installed adjacent to the photoconductive drum. The rotator rotates so that the selected developing device moves to the development area of the photoconductive drum (see U.S. Pat. No. 4,615,612)
The fixed-mode apparatus is unfavorable for miniaturization because a large-diameter photoconductive drum should be needed to secure sufficient space for a plurality of developing devices. Further, since distances from a portion where forming of the latent image of photoconductive material starts, i.e., the portion to be exposed, to a development portion of respective developing devices, are different, the damping amount of the potential of latent image is varied at respective development portions. For this reason, the density of the respective colors developed is not uniform and the color image finally formed on the sheet of paper bears stains, resulting in poor picture quality.
In the rotary type apparatus , at least one developing unit turns upside down when the rotator rotates, and the leakage of a developer contained therein may contaminate the interior of the apparatus. This is detrimental to the maintenance of the apparatus. Moreover, a high driving torque is required to drive the rotator. Therefore, when the rotator rotates, stable driving is difficult due to the occurrence of severe vibration and loud noise. This results in a limit in enhancing image quality.
U.S. Pat. No. 5,014,095 discloses another image forming apparatus which has a different mode from the above-discussed fixed and rotary type. This image forming apparatus, as shown in FIG. 1, comprises an elevation box 3 in which four developing units 2m, 2y, 2c and 2b are stacked. The box 3 is lowered or raised so that a developing unit selected on one side of photoconductive drum 1 moves to the development position of photoconductive drum 1. In this type of apparatus in which developing devices 2m, 2y, 2c and 2b are lowered or raised, the stains caused due to the difference of potential of the latent image between respective developers are removed and the contamination due to the leakage of developer is prevented. However, the mode is still disadvantageous in miniaturization because a sufficient space is needed for moving box 3 accommodating developing devices 2m, 2y, 2c and 2b. Further, a driving power of high torque must be used, which is a hinderance to the stable driving of the apparatus, and the enhancement of image quality is not so effective.